Owing to progress in digital communication networks such as the Internet in recent years, users of personal terminals can easily access network information.
In such digital communication networks, information is transmitted by digital signals. Even an individual user can copy music or movie data transmitted via the aforementioned digital communication network, and thereby can copy such data without degrading audio and/or image qualities.
Therefore, the copyright of the owner may be significantly infringed unless appropriate measures are taken for copyright protection when a copyrighted content such as music data or image data is transmitted over the digital communication network.
However, if copyright protection is given top priority, it may become impossible to distribute content data over the fast-growing digital communication network. This impairs an interest of the copyright owner, who can essentially collect predetermined copyright royalties for distribution of the copyrighted data.
Instead of the distribution over the digital communication network described above, distribution may be performed via record mediums storing digital data. In connection with the latter case, music data recorded on CDs (compact disks) on the market can be freely copied in principle onto magneto-optical disks (e.g., MDs) as long as copies are made only for the personal use. However, personal users performing digital recording or the like indirectly pay predetermined amounts in prices of digital recording devices or mediums as guaranty moneys to copyright owners.
In view of the fact that the music data copied from a CD to an MD is digital data, which does not substantially cause copy deterioration, devices and others are configured for copyright protection to prohibit further copying of the copied music data from the recordable MD to another MD.
In connection with the above, the public distribution itself of the content data such as music data and image data over the digital communication network is restricted by the public transmission right of the copyright owner, and therefore sufficient measures must be taken for the copyright protection in such distribution.
In the above case, it is necessary to prohibit unauthorized further copying of the content data such as music data or image data, which was once sent to the public over the digital communication network.
A data distribution system has been proposed for distributing content data over a digital communication network. In this data distribution system, a distribution server distributes content data to memory cards, which are data storage devices attached to terminal devices such as cellular phones. In this data distribution system, a public encryption key of the memory card, which is already certified by a certification authority, and its certificate are sent to the distribution server when requesting the distribution of the encrypted content data. After the distribution server confirms the reception of the certified certificate, the encrypted content data and a content key for decrypting the encrypted content data are sent to the memory card. When distributing the encrypted content data and the content key, the distribution server and the memory card generate session keys, which are different from those generated for other distribution processes. With the session keys thus generated, the public keys are encrypted, and the keys are exchanged between the distribution server and the memory card.
Finally, the distribution server sends the content key, which is encrypted with the public key peculiar to each memory card, and is further encrypted with the session key, as well as the encrypted content data to the memory card. The memory card stores the received content key and the encrypted content data in the memory card. In this operation, the license is stored in a license storage portion, which can ensure the safety.
When the encrypted content data recorded in the memory card is to be reproduced, the user connects the memory card to the reproduction terminal provided with a dedicated producing circuit, and thereby can reproduce the encrypted content data for enjoying it.
In the above system, usage rules are determined so that a content supplier or a copyright owner can instruct a manner of use in connection with reproduction and copying of the encrypted content data. The rules thus determined are distributed together with the content key so that each device can perform processing according to the usage rules.
The usage rules defines rules relating to copy/shift of the license between memory cards, rules such as restrictions on allowed times of reproduction in connection with output of the content key from the memory card, and rules relating to handling of reproduced contents.
In the data distribution system described above, the encrypted content data and the license are transmitted, e.g., between the distribution server and the memory card, or between the memory card and the reproduction terminal. The “licenses” generally represent the content key, usage rules, license ID for identifying the license and usage rules of contents already described. Such licenses are to be transmitted while ensuring sufficient security for the purpose of copyright protection.
In an operation of transmitting the license between devices, when ordinary transmission processing is being performed, the sender and the receiver mutually recognize the transmitted licenses, respectively, so that the license can be transmitted between the devices without any problem. However, when a failure such as power-down occurs in either of the devices or a communication path during the transmission of the license, the license may be lost during the transmission.
For the processing of, e.g., transmitting the license between the memory cards, the system is configured to prevent such a state that both the memory cards on the sender and receiver sides can simultaneously utilize the same license when storing the data, in view of the copyright protection, except for the case where the usage rules do not restrict the copying of the license. Thus, the license stored in the memory card on the sender side must be configured to become unavailable at the same as the output of the license to the memory card on the receiver side. In this configuration, such a state temporarily occurs that neither of the memory cards has stored the license in an available state. When the transmission processing is interrupted during the above state, the license, which is being transmitted, is lost. In the operation of receiving the license from the distribution server, the license may likewise be lost. In the case where the transmission of the license is interrupted, it is therefore important to determine whether the license is lost during the transmission thus interrupted, and to perform restoring or resending of the lost license in the optimum manner if the license was lost. For determining the loss of license, log information for specifying the interrupted transmission processing and the license must be stored efficiently in the memory card. It is also necessary that the memory card has a function of determining whether it has stored the license in question or not.
It can be reliably considered that the information transmission technology, which has been remarkably progressed in recent years, will further progress, and such progress will result in further advance in communication technology and increase in information amount. For these reasons, a data storage device of a large capacity, which can store various kinds of and a large number of content data, has been desired in the field of the data distribution system described above.
The data storage device of the large capacitance requires a data storage portion capable of storing a large number of licenses. When transmission of the license is interrupted in the above data distribution system, it is determined whether the license to be transmitted by the transmission thus interrupted is stored or not, and for this determination, search or retrieval is effected on the license storage portion, which can store a large number of licenses. However, it takes a long time for such retrieval processing, and this time increases with increase in number of the storable licenses.
According to a conventional system, the retrieval processing must be effected on the license storage portions one by one for the license determination, and the time required for the retrieval processing may cause a problem in the foregoing case.